Recently, with the widespread use of portable and cordless electronic equipment, the expectation has been also increasing for compact secondary batteries with lightweight and large energy density as a driving power source for such equipment. Furthermore, technology development of not only batteries used for small consumer goods but also large secondary batteries for electric power storages and electric vehicles, which require a long-time durability and safety, has been accelerated. From such a viewpoint, a non-aqueous electrolyte secondary battery having high voltage and large energy density, in particular, a lithium secondary battery is expected as a power source for electronic equipment, electric power storage and an electric vehicle.
A non-aqueous electrolyte secondary battery includes a positive electrode, a negative electrode, a separator and a non-aqueous electrolyte both interposed therebetween. A separator is mainly composed of a microporous polyolefin film. As a non-aqueous electrolyte, a liquid-state non-aqueous electrolyte (non-aqueous electrolyte solution) obtained by dissolving a lithium salt such as LiBF4 and LiPF6 in an aprotic organic solvent is used. As an active material for the positive electrode, lithium cobalt oxide (for example, LiCoO2) is used. Lithium cobalt oxide has a high electric potential with respect to lithium, is excellent in safety and is synthesized relatively easily. As an active material for the negative electrode, various carbon materials such as graphite are used. Non-aqueous electrolyte secondary batteries having such a configuration are made into practical use.
Since graphite used as an active material for a negative electrode can absorb one lithium atom per six carbon atoms theoretically, a theoretical capacity density of graphite is 372 mAh/g. However, the actual capacity density is reduced to about 310 to 330 mAh/g by a capacity loss due to the irreversible capacity. Therefore, it is basically difficult to obtain a carbon material capable of absorbing and releasing a lithium ion at this capacity density or more.
Then, in the circumstances where batteries with a larger energy density are demanded, silicon (Si), tin (Sn), germanium (Ge) and oxides or alloys thereof have been expected as a negative electrode active material having a large theoretical capacity density. Among them, Si and oxide of Si have been widely studied because they are inexpensive.
However, when Si, Sn, Ge and oxides or alloys thereof absorb lithium ions, the crystalline structure thereof is changed and the volume thereof is increased. When the active material largely expands at the time of charging, the contact failure between an active material and a current collector occurs. Consequently, the charge and discharge cycle lifetime is short. In order to address such a problem, the following proposals have been made.
For example, from the viewpoint of addressing a problem of the contact failure between the active material and the current collector due to expansion, a method for forming a thin film of an active material on the surface of a current collector has been proposed (for example, see Patent document 1). Furthermore, a method for forming an active material in a columnar shape and in an inclined state on the surface of a current collector has been proposed (for example, see Patent document 2). According to these proposals, stable current collection can be secured by strongly bonding an active material and a current collector to each other. In particular, space that is necessary and sufficient to absorb expansion is secured around the columnar active material in the latter case. Therefore, collapse of the negative electrode itself due to the expansion and contraction of the active material is prevented, and pressure stress to the separator and the positive electrode is reduced, and thereby, the charge and discharge cycle characteristic can be specifically improved.
However, when silicon oxide (SiOx(0<x<2)) is used as the active material, an irreversible capacity generated at the initial charge is very large. Therefore, when such a negative electrode is used as it is in combination with the positive electrode, a large portion of the reversible capacity of the positive electrode is consumed as the irreversible capacity. Therefore, in order to realize a large capacity battery by using silicon oxide as an active material for a negative electrode, it is necessary to compensate lithium from other than positive electrode.
Therefore, for compensating lithium, a large number of methods for providing metallic lithium to a negative electrode and allowing it to be absorbed in the negative electrode by a solid phase reaction have been proposed. For example, a method including a process of vapor-depositing lithium on the surface of a negative electrode and a process of storing has been proposed (for example, Patent document 3).
However, when an active material is formed by the method described in Patent documents 1 and 2, then the negative electrode is returned to the air, and then lithium is vapor-deposited on a surface of the negative electrode as described in Patent document 3, the active material may be reacted with water or oxygen in the air before lithium is vapor-deposited. Thus, the degree of oxidation of the active material is changed and it is displaced from the intended degree of oxidation. In an extreme case, the active material generates heat, thereby making handling difficult. Furthermore, when water is adsorbed on the surface of the active material, lithium does not easily diffuse into the active material even when lithium is vapor-deposited. Therefore, the vapor-deposited lithium may not be efficiently used for compensation of the irreversible capacity.    Patent document 1: Japanese Patent Unexamined Publication No. 2002-83594    Patent document 2: Japanese Patent Unexamined Publication No. 2005-196970    Patent document 3: Japanese Patent Unexamined Publication No. 2005-38720